Solar cells comprise an absorber layer where photons are absorbed and generate electron-hole pairs. The absorber layer of planar thin film solar cells needs to be thick enough for most of the incident photons to be captured but not so thick that all the charge carriers produced by photon absorption have either recombined or have been trapped before they reach the solar cell electrodes and generate photocurrent. The result is a difficult compromise between these two opposing effects and the efficiency of such cells is less than optimal.
Absorber layers may be comprised of materials such as silicon (microcrystalline and amorphous silicon), other silicon-based materials such as SiGe and carbon doped microcrystalline silicon, copper indium selenide (CIS), copper indium gallium selenide (CIGS), Cu(In,Ga)(S,Se)2 (CISSe), and various II-VI binary and ternary compounds. However, absorber layers are often defective materials with densities of recombination sites that have a significant effect on the efficiency of the solar device. There is a need to reduce the recombination of electron-hole pairs, also referred to herein as decay of charge carriers, in order to provide more efficient solar cells.
Furthermore, for amorphous silicon solar cells there is an additional problem called the Staebler-Wronski effect, which is a degradation of performance over time, and the degradation is greater for thicker amorphous silicon films. For example, a 300 nm amorphous silicon film may exhibit 10-12% light induced degradation in cell efficiency, moreover, the degradation increases exponentially to approximately 30% with increasing film thickness. However, light induced degradation of an amorphous silicon based solar cell depends not only on thickness, but also on the growth rate of the amorphous silicon, the deposition parameters, etc. Microcrystalline silicon films exhibit much less (1-2%) light induced degradation, even when the crystallinity fraction is low.
Improvements in solar cells have been made by creating three dimensional solar cell structures in which the absorber layer is formed over a non-planar surface, such as a surface with raised micron-scale pillars or ridges. This configuration allows for greater light absorption for a given absorber layer thickness, compared with a planar configuration. While promising results have been obtained for such solar cell structures, the fabrication of these cells is difficult on an industrial scale. There is a need for more manufacturable high efficiency solar cells and methods and apparatuses for forming the same.